The present invention relates generally to plasma systems and, more particularly, to a method for determining the endpoint of a plasma etch process.
In many processes for microelectronic device fabrication, a pattern defined by a lithographic technique is transferred through a layer of material formed on the surface of a substrate. Typically, the pattern is transferred by etching using a plasma. The term plasma, as used in this disclosure, refers to a partially ionized gas consisting of positively and negatively charged molecular species, as well as neutrals.
Plasma etching processes are typically performed in an apparatus such as a plasma reactor. Plasma reactors generally include a reaction chamber, a plasma generating system, a wafer holder and handling system and a gas delivery system (i.e., inlet, exhaust, and flow control). The term reaction chamber, as used in this disclosure, refers to the area within a plasma reactor where ionized gases physically and/or chemically interact with a material layer formed on the surface of a substrate.
A cross-sectional view of an example of a plasma reactor, called a parallel plate reactor 10 is shown in FIG. 1. Parallel plate reactor 10 includes two electrodes 11, 12 positioned parallel to each other in a reaction chamber 14. Substrates 15 with lithographically defined patterns (not shown) formed thereon are placed on the surface 12a of electrode 12. In a typical etching process using a plasma reactor such as a parallel plate reactor 10, gases are mixed and introduced into the reaction chamber 14. The mixed gases flow between electrodes 11, 12. An electric field applied between electrodes 11, 12 ionizes the gases and forms a plasma 13. The plasma 13 then etches the layer of material (not shown) formed on the surface of substrates 15 and transfers the lithographically defined pattern therethrough.
A problem associated with plasma etching processes is a difficulty in determining when the etch step has been completed. This difficulty occurs because plasma techniques are typically timed processes, based on predetermined etch rates. The predetermined etch rates are identified by performing a calibration step. Since the exact conditions (i.e., pressure, gas flow, electric field) used during the calibration step are typically not duplicated for the etch step, timed processes are inaccurate and only provide an estimate as to when the plasma etch process is completed.
In order to avoid the use of timed processes for determining the endpoint of an etch step, diagnostic techniques have been developed which analyze the plasma in the reaction chamber. One such technique, called optical emission spectroscopy, monitors the intensity of the optical emission in the plasma. The intensity of the optical emission is related to the concentration of molecular species in the plasma. The completion of the etch process is determined when a change in the intensity of the optical emission is observed. A change in the intensity of the optical emission is observed when the concentration of molecular species in the plasma changes as a result of etching through the top layer and into the underlying substrate. Optical emission techniques require the reaction chamber to be equipped with an optical port for monitoring the optical emission of the plasma. Optical ports are not universally available in production environments, which limits the use of optical emission techniques for plasma etch endpoint detection.
Other diagnostic techniques such as laser interferometry, ellipsometry and mass spectrometry have been utilized in laboratory environments to identify the endpoint of an etch process. However, these techniques are both expensive and difficult to implement. In addition, optical ports are required to monitor the plasma etch process using laser interferometry and/or ellipsometry. While optical ports are not required to perform mass spectrometry, the detectors used for such techniques are placed in the reaction chamber and are often corroded by the etchants used for etching the material layers, limiting the ability of the detectors to accurately detect the completion of the etch step.
Accordingly, techniques useful for determining when an etch step is complete and which do not rely on the optical emissions of the plasma or direct optical access to the substrate in the reaction chamber and which are not corroded by the chemical gases used for such processes, are sought.
U.S. Pat. No. 5,877,407 assigned to the assignee of the present invention describes a method for determining the endpoint of a plasma etch process using acoustic cells. For the purpose of this description, the endpoint of the plasma etch process refers to when a first material layer formed on the surface of a substrate is etched through its thickness to its interface with an underlying material layer. The acoustic cell is configured to have a transmitter and a receiver located at opposite ends of a conduit. The transmitter and the receiver are acoustically matched transducers, which preferably operate in the kilohertz range, such as a transducer of lead-zirconate-titanate crystal. At least a portion of a gas stream from a reaction chamber of a plasma reactor flows through the acoustic cell during the plasma etch process with the pressure at which the gas stream flows in the acoustic cell preferably at least about 10 torr.
As the gas stream from the reaction chamber flows in the acoustic cell, acoustic signals are periodically transmitted from the transmitter to the receiver and the velocity of such acoustic signals is determined. The acoustic signals are periodically transmitted at intervals of at least about 20 hz (hertz), for the duration of the etch process. The acoustic signals, when transmitted, preferably travel a distance less than about 6 inches in the acoustic cell and are transmitted at a frequency within the range of about 50 kilohertz to about 500 kilohertz.
According to U.S. Pat. No. 5,877,407, the velocity of an acoustic signal is related to the average molecular weight of the gas stream according to the expression                               v          s                =                                            γ              ⁢                              xe2x80x83                            ⁢              RT                        M                                              (        1        )            
where vs is the velocity of the acoustic signals, R is the universal gas constant (8.3143 J/mol K), T is the temperature in degrees Kelvin, M is the average molecular weight of the gas, and xcex3 is the ratio of the average specific heat at constant pressure to the average specific heat at constant volume (Cp/Cv). Thus, at constant temperatures, the velocity of an acoustic signal changes as the average molecular weight of the gas changes. For example, if the average molecular weight of the gas decreases, the velocity of acoustic signals transmitted through the gas increases.
As the first material layer formed on the surface of a substrate is etched, the average molecular weight of the gas flowing in the acoustic cell does not vary significantly. Thus, from equation (1), the velocity of the acoustic signals transmitted through the gas exhausted from the reaction chamber as the first material layer is etched approximates a constant value. For the purpose of this description, the acoustic signals transmitted through the gas exhausted from the reaction chamber when the first material layer formed on the surface of the substrate is etched, have a first velocity (or reference velocity).
When the first material layer is etched through its thickness to its interface with the underlying material layer, the average molecular weight of the gas flowing into the acoustic cell changes. The change in the average molecular weight of the gas changes the speed at which the acoustic signals are transmitted through the gas in the acoustic cell. Thus, the first velocity determined for the acoustic signals transmitted through the gas as the first material layer is etched changes to a second velocity associated with reaching the interface of the underlying material layer. The second velocity for the acoustic signals differs from the first velocity by more than 1%. The etch process endpoint occurs when the second velocity for the acoustic signals transmitted through the gas, is observed.
Since acoustic signals are not easily transmitted through gases at low pressures (less than about 10 torr), the gas stream introduced into the acoustic cell first flows through a compressor. The compressor, preferably a vacuum pump, compresses the gas stream thereby increasing its pressure to a pressure greater than about 10 torr.
While the method of U.S. Pat. No. 5,877,407 has been shown to be effective in determining endpoints in a plasma etch process, applicants have found that the method does not provide consistently ideal results in endpoint detection.
The present invention is an improvement to the basic acoustic cell endpoint detection method of U.S. Pat. No. 5,877,407. In particular, the present invention provides a more precise detection of plasma etch endpoints based upon applicants"" discovery that the acoustic velocity relationship as set forth in equation (1) above assumes standard conditions, e.g., constant pressure of the gas in the acoustic cell. Applicants have found that variation in pressure can affect acoustic cell response and thereby cause variation in endpoint detection.
The present invention overcomes the above described detriment, in a preferred embodiment, by exhausting all of the gas stream through the acoustic cell, rather than just a portion, and using an exhaust control valve to regulate the pressure in the acoustic cell to a preselected, high value such as up to about 100 torr. A first pump, such as a vacuum pump, is used to pressurize the gas stream entering the acoustic cell from the plasma chamber. A controllable valve is fitted to an exhaust port of the acoustic cell and serves to regulate the outflow of the gas stream from the acoustic cell. A pressure monitor is coupled to the acoustic cell and to the controllable valve. The pressure monitor provides signals to the valve to adjust the gas stream outflow in a manner to maintain a substantially constant pressure in the cell. Constant pressure regulation in the acoustic cell is believed to produce more precise endpoint detection than the unregulated pressure method.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.